a) Field of the Invention
The present invention is directed to a compressor device, an expander device, and a compander system and to a corresponding transmission method.
b) Description of the Related Art
Compander systems are known, for example, from the sphere of cassette recorders or analog FM radio transmission systems for suppressing noise and increasing the dynamic range. A digital implementation of a system of this kind was described by Peissig et al. in “Digital emulation of analog companding algorithms for FM radio transmission”, DAFX-04 Conference on Digital Audio Effects, pages 285-290, Naples, Italy, 2004. For this purpose, analog circuits were replaced by a digital signal processor.
A cassette recorder or an analog FM radio transmission typically has an audio dynamic range of only 50-70 dB. As a result of this reduced dynamic range, audible noise cannot be prevented. Audio compressors are used during recording or during transmission to suppress perception of this noise. The compressor reduces the dynamic range of the input signal so that all signal amplitudes lie above the noise threshold. During playback or FM reception, an expander restores the original dynamic range of the signal using attenuation depending on the signal amplitude. Accordingly, the dynamic range is expanded and the noise level is reduced simultaneously. A system of this kind is known as a compander (compression and expansion) and performs a time-variable processing for audibly changing the processed signal.
FIG. 6 shows a block diagram of a conventional compander system. The system has a pre-emphasis unit PRE, a multiplier unit M1, a level detecting and control unit LDC, and a divider unit D1. The compressed signal is transmitted over a channel Ch and the reception side or expander side has, likewise, a level detecting and control unit LDC and also a de-emphasis unit De. The level detecting and control unit LDC serves to derive a gain factor from the signal level so that the dynamic range of the input signal in the compressor is reduced by amplifying weak signals, while strong signals remain unchanged. In the expander, the process is carried out in reverse on the reception side in that the same gain factors are derived and the signal is divided by these gain factors by the divider unit D1. The channel noise is reduced for most of the weak signals, which results in an improved audio quality. In addition, pre-emphasis filtering can be implemented on the transmission side, which slightly amplifies high frequencies, while the inverse operation of de-emphasis filtering is implemented on the reception side so that the channel noise is attenuated at higher frequencies.
Further, a compression/expansion can be carried out in different sub-bands. This has the advantage that compression parameters such as the time constants for the level detection of the different sub-bands, for example, can be selected individually so that a better balance can be found between noise reduction and the emergence of compression artifacts. Further, not only is the remaining noise at the expander output reduced, but the spectrum of noise is shaped substantially corresponding to the signal so that much of the noise falls below an audibility threshold.
“Spectral amplitude warping (SAW) for noise spectrum shaping in audio coding”, IEEE International Conference on Acoustics, Speech, and Signal Processing, Volume 1, pages 335-338, April 1997, shows a block-by-block transformation of the input signal into the frequency domain, application of a non-linear mapping for every frequency bin, and a back-transformation into the time domain using overlap-and-add. Both the transmitter and the receiver operate in this way and differ only in the non-linear function that is used. In the transmitter, the kth frequency X(k) is mapped by
                    f        k            ⁡              (                  X          ⁡                      (            k            )                          )              =                            X          ⁡                      (            k            )                                                          X            ⁡                          (              k              )                                                    ⁢                                              X            ⁡                          (              k              )                                                          α          ⁡                      (            k            )                                ,          ⁢      0    <          α      ⁡              (        k        )              ≤    1    ,whereas in the receiver the inverse equation is used, i.e., the exponent is replaced by 1/α(k).
Spectral amplitude warping (SAW) can improve the perceived quality of audio coding systems that do not shape noise in any adequate way by implementing pre-processing steps or post-processing steps in an existing coder. It may be assumed that this method is also suitable for a noisy analog communications link. However, the main disadvantage of this method is the latency at both ends due to the block-based processing.